Moths damaging Douglas-fir trees across western Montana

Moths described as voracious feeders are damaging Douglas-fir trees throughout western Montana, according to a news release from the Montana Department of Natural Resources and Conservation.

The department said the trees are being damaged by an outbreak of Douglas-fir tussock moth. It is localized to the historic outbreak regions of Missoula, Plains, Thompson Falls, Polson, Kalispell, Columbia Falls and the Flathead Lake region at this time.

The DNRC and the U.S. Forest Service are monitoring the scope and extent of damage.

For the previous one to two years, the Douglas-fir tussock moth has damaged the tops of ornamental Colorado blue spruce, which is typical prior to noticeable damage in Douglas-fir forests, the state agency said.

The moths can cause alarming damage to trees, the department said, but a virus naturally builds up by the third year of an outbreak and controls the outbreak. Insects in the highly visible patches are estimated to be in their second year of activity, although surveys are underway to identify areas where the outbreak is in earlier stages and damage may continue in the coming years.

Healthy trees may rebound from the defoliation, but young, densely stocked or drought-stressed trees may die, the DNRC said. Trees growing in overstocked stands will likely suffer greater damage, and young or suppressed trees often do not have enough nutrient reserves to withstand the damage. More trees may die if defoliation continues or bark beetles move into the stand.

The Forest Service reports the Douglas-fir tussock moth “can be one of the most damaging of western defoliators.” The agency says the moth has a one-year life cycle and overwinters as eggs.

Adult male moths are a non-descript, gray-brown color, with feathery antennae, and females are flightless, with rudimentary wings and a large abdomen.

Landowners should consider thinning stands to reduce competition for light, water, and nutrients, DNRC said. Some areas may not yet be showing evidence of impacts from the moth but could be damaged in the next year or two. Spraying insecticides is not typically warranted unless defoliation is intolerable on individual high value trees. Spray must be applied when the moths are still actively feeding in spring or early summer.

While the Douglas-fir tussock moth is not dangerous to the public, the hairs on the moth can cause an allergic reaction in humans known as “tussockosis.” The most common symptoms are irritation to the skin and lungs. Avoid handling any larvae and areas where the outbreak may be more severe, the DNRC said.

“The situation surrounding the Douglas-fir tussock moth is still evolving,” said Amy Gannon, entomologist for the Montana Department of Natural Resources and Conservation. “The good news is Montana has a lot less Douglas-fir tussock moth than surrounding states and people are likely not having sustained contact with the larvae.”

The western spruce budworm is also active in many of the outbreak areas. The budworm can be distinguished from the Douglas-fir tussock moth by observing the larvae. The budworm larvae are hairless and have cream-colored dots along their backs, whereas the Douglas-fir tussock moth larvae have very distinguishable tufts of hair along their backs, the DNRC said.